In the optical transmission network of WSON (Wavelength Switched Optical Network), which is also called WDM ASON (Wavelength-Division Multi-plexing Automatically Switched Optical Network), it is required to design appropriate RWA (Routing and Wavelength Assignment) implementation scheme to realize SPC (Soft Permanent Connection). Due to the nondeterministic polynomial characteristics of RWA, the route R process and wavelength allocation WA process included in RWA are to be implemented in two steps. The WA wavelength allocation process of the current WSON transmission network is not fully wavelength switched generally; due to the confine imposed by the limitation of block crossing on transmission node, multiple alternative routes should be provided in the computing process of R routing so as to provide an alternative route and to proceed a new WA wavelength allocating process after a failure in WA analysis on a selected route. In RFC4655 and RFC4657, the functions and architectures of PCE (Path Computation Element) as the path computing unit of GMPLS (Generalized Multiprotocol Label Switching) are described emphatically, and the alternative routing is the very function that PCE needs to possess under block crossing conditions.
Generally, route computing should satisfy constraining conditions distributed by a managing plane, wherein the constraining conditions generally comprise: must-be-avoided nodes, links, SRLG (Shared Risk Link Group), and must-be-passed-through nodes, links, SRLG. Accordingly, it is required that the K lines of alternative routes provided by the ASON control plane must fully satisfy the constraining conditions on route computing that are distributed by the managing plane. Furthermore, when the control plane computes the route under the must-be-passed-through constraining conditions, the computing is generally implemented in a way of segment-wise computing, i.e., to divide an entire route to be computed sequentially from the SPC source node to the destination node into several subsegments based on the nodes, links and SRLG that must be passed through, as shown in FIG. 1. When computing the alternative routes of each subsegment, the conventional practice is to shield all the network resource topology that has been passed through by all the computed alternative routes of all the subsegments (except the source node and the destination node of the subsegment), i.e., to take all the nodes, links and SRLGs that have been passed through by all the alternative routes of all the subsegment as must-be-avoided constraining conditions and also to take the must-be-passed-through nodes, links and SRLGs other than the subsegment as the must-be-avoided constraining conditions of the route computing of the subsegment. The result of that is that because the must-be-avoided network resources become increasingly more, the number of alternative routes computed for the subsequent subsegment to be computed becomes increasingly less.